1. Field of the Invention
The present invention relates to an all-solid-state lithium secondary cell using a lithium ion-conducting solid electrolyte, and a method of manufacturing the cell.
2. Background Art
With downsizing of electronic equipment, there has been an increasing demand for cells having high energy density as its main power supply or backup power. Among these, a lithium ion secondary cell is drawing attention because of its higher voltage and energy density than a conventional cell using an aqueous solution as the electrolytic solution thereof. The lithium ion secondary cell includes: an oxide, such as LiCoO2, LiMn2O4, and LiNiO2, as a positive electrode active material; carbon, an alloy of Si and the like, and an oxide such as Li4/3Ti5/3O4, as a negative electrode active material; and a solution dissolving a lithium salt in an organic solvent such as carbonate or ether as an electrolytic solution. For the electrolytic solution, a flammable substance classified in dangerous substance Class 4 is used. Additionally, deterioration of safety and damages to the equipment incorporating the cell caused by leakage of the electrolytic solution is concerned about.
To make up for such drawbacks of a lithium ion secondary cell, an all-solid-state lithium secondary cell using a solid electrolyte in place of an electrolytic solution is studied. The all-solid-state lithium secondary cell has a laminate including a solid electrolyte disposed between positive and negative electrodes as an electricity-generating element. External current collectors are disposed at both ends of the laminate so as to support the side faces of the laminate. This cell can avoid such a problem of the above-mentioned leakage because its electrolyte is not a liquid. Thus, the cell can be mounted directly on a wiring board, and the equipment incorporating the cell can be greatly downsized.
However, the current density of the all-solid-state lithium secondary cell is smaller than that of a lithium secondary cell using a non-aqueous solution. To overcome this problem, Japanese Patent Unexamined Publication No. H06-231796, for example, proposes a method of extending the electrode area by sandwiching a solid electrolyte between positive and negative electrodes, and laminating the combination. This method allows the cell to easily be mounted on a circuit board because the cell is shaped into substantially a rectangular parallelepiped, as well as improving the capacity and current density of the cell.
Each of the external current collectors are formed by applying a paste made of an electrically-conductive powder and a thickener onto, among all of the side faces of the laminate, each side of the laminate to which the end face of each electrode is exposed. A cupper powder, for example, is used as the electrically-conductive powder. Glass frit, for example, is used as the thickener. Thus, the external current collectors have a rounded shape. Because the external current collectors are formed in this manner, the external current collectors at the edges or along the ridges are likely to be thinner when the laminate is shaped into substantially a complete rectangular parallelepiped. In an extreme case, the edges or ridges of the cell can be exposed. The laminate having a portion which is partly exposed from the external current collectors leads to poor solder wettability of the external current collectors when the laminate is mounted on a circuit board, whereby causing mounting failures. The edges and ridges which are less resistant to external stress sometimes make weaker the structure of the laminate when it is dropped or the circuit board is warped in the reflow-soldering process.